It is well established that the processing of amyloid precursor proteins (APP) leads to the generation of A[unreadable], a protein important in Alzheimer's disease, but the normal functions of APP in a synapse are unknown. Cleavages of APP are known to be altered by interactions with extracellular ligands. We and others have recently found that proteins of the extracellular matrix (ECM) are ligands that affect the trafficking of APP. Recently, we made a key discovery that APP interacts with Reelin and a3[unreadable]1-integrin using co- immunoprecipitation in the brain. Extracellular matrix proteins and integrins are known to be involved in dendritic neuronal development, and we hypothesize that APP is also involved in neurite outgrowth and synaptogenesis. In support of this idea, we found that APP and a3[unreadable]1 integrin are expressed in synapses and highly expressed during the peak of synaptogenesis. Furthermore, we found that APP overexpression increased neurite outgrowth, whereas APP shRNA decreased neurite outgrowth. Importantly, we also found that a3[unreadable]1 integrin antibody decreased neurite outgrowth and prevented the effects of APP on neurite outgrowth. Further, we found that dendritic arborization was impaired in Reeler mice, supporting the idea of an important role for Reelin in neurite outgrowth. Interestingly, APP knockdown prevented the effects of Reelin on neurite outgrowth in hippocampal neurons. These results suggest that APP promotes neurite outgrowth, a process that we propose depends on the physical interactions among APP, Reelin, and a3[unreadable]1 integrin. Thus, we will determine whether interactions between APP and Reelin, APP and a3[unreadable]1 integrin, or APP, Reelin and integrins work synergistically or competitively with one another to alter neurite outgrowth. The goal of this application is to explore potential new interactions (Reelin-APP or a3[unreadable]1 integrin-APP), test the regulation of known interactions (Reelin- a3[unreadable]1 integrin) and determine what effects these interactions have on neuronal development. We will utilize molecular/biochemical and cell biology approaches to determine the mechanism by which APP coordinates Reelin- a3[unreadable]1 integrin to regulate dendritic neurite outgrowth. In Aim 1, we will determine the biological significance of APP alone as well as its interaction with integrins on dendritic neurite outgrowth. In Aim 2, we will determine the role of APP-binding proteins (e.g., Reelin, integrins) and a triple complex consisting of these proteins in modulating dendritic neurite outgrowth. Through these experiments, we will address the hypothesis that APP and its binding partners are critical for normal neuronal development and connection of synapses, and aids in correct neuronal positioning, which has implications for the extensive synapse loss and cognitive decline seen in Alzheimer's disease. PUBLIC HEALTH RELEVANCE: The proposed experiments are important for understanding the physiological actions of APP in normal tissue. The goal of this application is to explore potential new interactions (Reelin-APP), test the regulation of known interactions (Reelin- a3[unreadable]1 integrin) and determine the effect these interactions have on APP trafficking. Additionally, these experiments will address the hypothesis that ligand-binding of APP is critical for normal neuronal development, connection of synapses, and aids in correct neuronal positioning, which has implications for the extensive synapse loss and cognitive decline seen in Alzheimer's disease.